In oilfield operations, drilling into rock requires relatively large power levels and forces that are usually provided at the drilling rig by applying a torque and an axial force through a drill string to a drill bit. The lower portion of the drill string in a vertical well includes (from the bottom up) the drill bit, bit sub, stabilizers, drill collars, heavy-weight drill pipe, jarring devices and crossovers for various thread forms. The bottom hole assembly, hereinafter referred to as the BHA, provides force, the measure of which is referred to as “weight-on-bit”, to break the rock and provide the driller with directional control of the well. In conventional drilling, the BHA is lowered into the wellbore using jointed drill pipes or coiled tubing. Often the BHA includes a mud motor, directional drilling and measuring equipment, measurements-while-drilling tools, logging-while-drilling tools and other specialized devices. A simple BHA consisting of a drill bit, various crossovers, and drill collars is relatively inexpensive, costing a few hundred thousand US dollars, while a complex BHA costs ten times or more than that amount.
The drill bit section of the BHA is used to crush or cut rock. A dull bit may result in failure to progress and must be replaced. Most drill bits work by scraping or crushing the rock, or both, usually as part of a continuous circular motion in a process known as rotary drilling. During rotary drilling cuttings are removed by drilling fluids circulated through the drill bit and up the wellbore to the surface.
The use of coiled tubing with downhole mud motors to turn the drill bit to deepen a wellbore is another form of drilling, one which proceeds quickly compared to using a jointed pipe drilling rig. By using coiled tubing, the connection time required with rotary drilling is eliminated. Coiled tube drilling is economical in several applications, such as drilling narrow wells, working in areas where a small rig footprint is essential, or when reentering wells for work-over operations.
Many drilling operations require direction control so as to position the well along a particular trajectory into a formation. Direction control, also referred to as “directional drilling,” is accomplished using special BHA configurations, instruments to measure the path of the wellbore in three-dimensional space, data links to communicate measurements taken downhole to the surface, mud motors, and special BHA components and drill bits. The directional driller can use drilling parameters such as weight-on-bit and rotary speed to deflect the bit away from the axis of the existing wellbore. Conversely, in some cases, such as drilling into steeply dipping formations or due to an unpredictable deviation in conventional drilling operations, directional-drilling techniques may be employed to ensure that the hole is drilled vertically.
Direction control is most commonly accomplished through the use of a bend near the bit in a downhole steerable mud motor. The bend points the bit in a direction different from the axis of the wellbore when the entire drill string is not rotating. By pumping mud through the mud motor, the bit rotates though the drill string itself does not, allowing the bit alone to drill in the direction to which it points. When a particular wellbore direction is achieved, the new direction may be maintained by then rotating the entire drill string, including the bent section, so that the drill bit does not drill in a direction away from the intended wellbore axis, but instead sweeps around, bringing its direction in line with the existing wellbore. As it is well known by those skilled in the art, a drill bit has a tendency to stray from its intended drilling direction, a phenomenon known as “drill bit walk”. Drill bit walk results from the cutting action, gravity and rotation of the drill bit as well as irregularities of the formation being drilled. It is desirable to eliminate or at least minimize the drill bit walk to ensure that the drilling operation proceeds in the desired direction. While drill bit walk is generally undesirable, drill bit walk which is controlled could produce an intentional and favorable deviation from the established direction of drilling.
Most boreholes are nearly vertical and not particularly deep. In such wells, standard wireline cables are capable of carrying logging tools and other equipment to a desired depth. However, the scarcity of petroleum has resulted in the desire to explore formations which are more difficult to reach. Therefore, with ever increasing frequency, boreholes are extremely deep and have high inclination angles. For many years, drill pipe and coiled tubing have conveyed drilling bit and drilling equipment into the wellbore. Once at the required downhole location, the equipment is expected to perform complex tasks that often need to be monitored and controlled in real time at a surface rig site far from the wellbore.
It is desirable to have alternative conveyance technologies available in order to explore deeper and more difficult wells. One such technology may be autonomous drilling robots that are not connected to surface equipment using drill pipe, coiled tubing or other means.
If drilling robots are to be developed that use conventional rotational drilling techniques, the drilling robots must be able to support both drilling reaction torque and thrust force. If the drilling robots cannot counteract the reaction torque, the drilling robots would commence to rotate in the wellbore thereby reducing efficiency of the drilling operation. Designing a drilling robot that counters reaction torque is even more difficult for a well with a small borehole. A low rate of penetration of the drilling robot in the borehole would result in reduced torque on the drilling robot. However, at higher rates of penetration, e.g., using the same rotational velocity as employed in conventional drilling techniques, it can be expected that torque will be a problem for the robot.
A device for controlling torque while drilling a borehole is disclosed in U.S. Pat. No. 5,845,721 to Robert Charles Southard, whose invention includes a tubular drill string with a motor for generating a rotary force. The device further includes an inner drilling device adapted to the motor means and an outer drilling device concentrically arranged about the inner drilling device. Southard's device includes a planetary gear system adapted for imparting the rotation generated from the motor to the outer drilling device. A shaft extending from the motor is operatively connected to the inner drilling device, and the shaft has a plurality of shaft splines thereon formed to cooperate with the planetary gear system. Due to the particular configuration of the planetary gear system, the inner and outer drilling devices rotate in opposite directions. The inner and outer drill bits have a fixed gear relation resulting in a rotation of the inner and outer drill bits at a constant relative speed.
A drilling device is disclosed in US Patent Application Publication Number 2004/0011558 A1 to Sigmund Stokka, whose invention includes a method of introducing instruments or measuring equipment or tools into formation of earth's crust or other solid material by means of a drilling device, material being liberated by rotation of a drill bit, and the liberated material thereafter flowing, or being pumped, past or through the drilling device. Stokka's method includes absorbing the reaction torque produced by the drill bit's rotary moment of inertia by alternating the direction of rotation of the drill bit.
From the foregoing it will be apparent to those skilled in the art that there is a need for a remotely controlled drilling robot that can drill a borehole or a lateral deviation from an existing borehole in the oilfield and for such a drilling robot to eliminate or control the drilling reaction torque and thrust force applied to the attached drilling module. Furthermore, there is a need for an improved method to eliminate, reduce or manage the reaction torque from the drill bit to the robot. Furthermore, there is a need for an improved method for controlling drill bit walk that is caused by reaction torque from the drill bit either for the purpose of ensuring controlled straight-ahead drilling using mechanical geostationary reference or to steer the drilling operation in a new direction.